The presence of cytoplasmic vacuoles has been noted in primate and mouse embryos produced in vivo as well as in bovine embryos produced in vitro. These vacuoles frequently contain cellular debris resulting from autophagy or ingestion of embryonic cell fragments. It has been suggested that vacuoles are indicative of abnormal or delayed developmental differentiation. Because compact morulae produced in the IVPS culture treatment exhibited an increased volume density of vacuoles relative to embryos in the other treatment groups, the IVPS embryos may be more compromised in their development.
Compact morulae produced in the IVPS treatment also had an increased cytoplasmic-to-nuclear ratio compared to those produced in vivo. This resulted from both an increase in the volume density of cytoplasm and a decrease in the volume density of nuclei. The increased volume density of cytoplasm observed in morulae produced in vitro may result from incomplete compaction. Alternatively, the increased volume density of cytoplasm may have resulted from the occurrence of fewer cell divisions. Compact morulae produced in vitro had fewer cells than those produced in vivo. A decrease in the volume density of nuclei would be consistent with the presence of fewer blas-tomeres in compact morulae from the IVPS treatment.
Embryos produced in vitro have been reported to have a reduced degree of compaction and cell-to-cell coupling compared to those produced in vivo. If this were true, it might be expected that a greater volume density of intercellular space would have been present in embryos produced in vitro. However, in the present study, no differences were found in the volume densities for intercellular space in embryos from any of the four treatments. This discrepancy may be a result of differences in methods used for embryo production or for assessment of intercellular space.